As described in the Floodplain Development Manual (Reference 1), the Floodplain Risk Management Process entails four sequential stages:

• Stage 1: Flood Study
• Stage 2: Floodplain Risk Management Study
• Stage 3: Floodplain Risk Management Plan
• Stage 4: Implementation of the Plan

The Rose Bay Catchment Floodplain Risk Management Study constitutes the second stage in the process. The Flood Study stage was completed in September 2010 with publication of the Rose Bay Catchment Flood Study (Reference 2). A combination of hydrologic and hydraulic models was used in that study to determine design flood levels for the Rose Bay catchment.

This study superseded a previous Catchment Management Study (Reference 3) completed in 1991.

A rainfall-runoff approach was adopted due to the absence of long term historical flood data. This approach involved setting up a DRAINS hydrologic and hydraulic computer model that simulated flow both in the pipe system and as overland through private property and along roads. The DRAINS model covered the entire catchment. A two-dimensional (2D) SOBEK computer model was established in the lower reaches to convert the upstream flows obtained from DRAINS into flood levels and velocities.

Design rainfall data obtained from Woollahra Council and design temporal patterns from Australian Rainfall and Runoff (Reference 4) were obtained and used in a DRAINS model to determine design event flows. The SOBEK model was then used to determine design flood levels in the lower catchment. The downstream regions of the Rose Bay catchment were predominately influenced by catchment flows and only slightly by elevated water levels in Rose Bay. Sensitivity analyses were undertaken on the DRAINS and SOBEK model results.